1. Field of the Invention
This invention pertains to grinding mills and, more particularly, the invention pertains to a grinding mill with means for monitoring changes in the charge load of a grinding mill.
2. Description of the Prior Art
In the prior art, grinding mills are well known for grinding a variety of ores. It is desirable to monitor the amount of change in the charge load within a rotary grinding mill compartment. This is especially true for autogenous mills or semi-autogenous mills. As is apparent to those in the art, autogenous mills are mills where the ore itself is the grinding medium. In semi-autogenous mills, the grinding action is supplied by the ore and an additional medium such as steel balls. Since the inception of autogenous grinding mills, the art has been looking for means for measuring the amount of charge load in the mill compartment.
One method of measuring the amount of charge load in a grinding mill compartment is to measure the amount of power consumed by the grinding mill during operation. As the charge load increases, the power requirements for the mill increases. However, one very important phenomena occurs which makes monitoring of power an ineffective method of determining the amount of charge load in the mill compartment. For example, as the charge load approaches approximately 40 to 50% of the capacity of the mill, the power requirements for the mill actually level off and decrease. This is attributable in part to the fact that the tumbling action of the material within the mill compartment may assist in rotation of the mill. An accurate determination of the power requirements is important since the load is fed to the mill according to the power requirements. That is, a hard material is fed at slow rates since a high power requirement is needed to grind the hard material. A softer material is fed at a faster rate. Material is discharged continuously from the mill after it is ground. Therefore, the variation of charge in the mill compartment can occur continuously throughout operation of the mill.
Other methods of determining changes in charge load within the mill include measuring pressure of a lubricant between the mill trunnion and support bearing. However, this is not an accurate measure of the load in the compartment since the pressure of the hydrodynamic lubricant between the trunnion and the bearing is in part a function of the lubricant temperature. Another method for determining the charge load was to place load cells underneath main bearing assemblies. However, this is not successful since not all of the load is taken up by the load cell and some is taken up by the bearing base. As a result, it is not readily distinguishable what percentage is taken up by the load cells. Therefore, the amount measured by the load cells cannot be reasonably correlated with the amount of charge load within the mill compartment.